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SNS topic publicity has Allow and NotAction simultaneously

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Metadata

Id: cloudformation-aws-sns-topic-publicity-has-allow-and-not-action-simultaneously

Provider: AWS

Platform: CloudFormation

Severity: Medium

Category: Access Control

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Description

SNS topic policy statements must not combine Effect: "Allow" with a NotAction element. Using NotAction with Allow effectively permits all actions except the ones excluded and can unintentionally grant broad access to the topic.

Check AWS::SNS::TopicPolicy resources’ Properties.PolicyDocument.Statement entries. Any statement with Effect: "Allow" must include an explicit Action (for example, sns:Publish) rather than NotAction. Resources containing a statement where Effect: "Allow" and NotAction is present will be flagged. To block specific actions, use Effect: "Deny" with NotAction, or enumerate allowed actions explicitly in Action.

Secure configuration with explicit Action:

MyTopicPolicy:
  Type: AWS::SNS::TopicPolicy
  Properties:
    PolicyDocument:
      Version: '2012-10-17'
      Statement:
        - Effect: Allow
          Principal:
            AWS: arn:aws:iam::123456789012:root
          Action:
            - sns:Publish
          Resource: arn:aws:sns:us-east-1:123456789012:my-topic

Compliant Code Examples

AWSTemplateFormatVersion: 2010-09-09
Description: A sample template
Resources:
  mysnspolicy:
    Type: AWS::SNS::TopicPolicy
    Properties:
      PolicyDocument:
        Id: MyTopicPolicy
        Version: '2012-10-17'
        Statement:
        - Sid: Mystatementid
          Effect: Allow
          Principal:
            AWS: !GetAtt myuser.Arn
          Action: sns:Publish
          Resource: "*"
      Topics:
      - !Ref mytopic

{
  "Resources": {
    "mysnspolicy": {
      "Type": "AWS::SNS::TopicPolicy",
      "Properties": {
        "PolicyDocument": {
          "Id": "MyTopicPolicy",
          "Version": "2012-10-17",
          "Statement": [
            {
              "Sid": "Mystatementid",
              "Effect": "Allow",
              "Principal": {
                "AWS": "myuser.Arn"
              },
              "Action": "sns:Publish",
              "Resource": "*"
            }
          ]
        },
        "Topics": [
          "mytopic"
        ]
      }
    }
  },
  "AWSTemplateFormatVersion": "2010-09-09T00:00:00Z",
  "Description": "A sample template"
}

Non-Compliant Code Examples

AWSTemplateFormatVersion: 2010-09-09
Description: A sample template
Resources:
  mysnspolicy:
    Type: AWS::SNS::TopicPolicy
    Properties:
      PolicyDocument:
        Id: MyTopicPolicy
        Version: '2012-10-17'
        Statement:
        - Sid: MyStatementId
          Effect: Allow
          NotAction: "s3:DeleteBucket"
          Resource: "arn:aws:s3:::*"
        - Sid: MyStatementId2
          Effect: Allow
          NotAction: "iam:*"
          Resource: "*"
      Topics:
      - !Ref mytopic

{
  "AWSTemplateFormatVersion": "2010-09-09T00:00:00Z",
  "Description": "A sample template",
  "Resources": {
    "mysnspolicy": {
      "Type": "AWS::SNS::TopicPolicy",
      "Properties": {
        "PolicyDocument": {
          "Id": "MyTopicPolicy",
          "Version": "2012-10-17",
          "Statement": [
            {
              "NotAction": "s3:DeleteBucket",
              "Resource": "arn:aws:s3:::*",
              "Sid": "MyStatementId",
              "Effect": "Allow"
            },
            {
              "Sid": "MyStatementId2",
              "Effect": "Allow",
              "NotAction": "iam:*",
              "Resource": "*"
            }
          ]
        },
        "Topics": [
          "mytopic"
        ]
      }
    }
  }
}